The present invention relates to electronic power supplies with overload protection from frequent, intermittent or partial shorting of the power supply load. More specifically, the present invention relates to regulated direct current power supplies for providing a high level driving voltage for electrostatic precipitators.
Electrostatic precipitators are commonly employed to remove particulates from an air stream. In conventional precipitators, an ionizer is used to produce an electrostatis field to charge the contaminating particles. The charged particles are then accumulated in one or more collecting cells having plates or grids across which a voltage is applied to attract the charged particles. Such collecting cells are operated at high voltages, typically, at several thousand volts.
Particulates accumulating in the collector cells may cause frequent arcing or shorting of the high voltage power supply. In some situations, a low, but non-zero resistance current path may be created in the collecting cell. In other situations, virtually continuous grounding of the high voltage collector plates may occur. These effects can create high transient voltages or longer duration current surges which may damage the power supply circuits. Accordingly, it is an object of the present invention to provide a power supply for electrostatic precipitators with effective, automatic protection against frequent short circuiting (arcing) and long duration overloads.
In the prior art, fuses or circuit breakers have been used as overload protection for electrostatic precipitators. These devices have disadvantages due to the need to reset or replace them in the event of arcing, due to their lack of a graded response to varying types overload conditions encountered, and due to their typically slow response times. Also, frequent arcing may create a high level of noise.
Accordingly, it is an object of the present invention to provide automatic, fast acting circuit arcing control and overload protection which is responsive to the duration and degree of overload.
It is known in the art to employ ferro-resonant circuits in the high voltage transformer of a power supply for an electrostatic precipitator. Such a device is shown in FIG. 1. The circuit consists of a ferro-resonant transformer and a resonating capacitor C.sub.1. The transformer core provides a closed magnetic loop. The remaining circuitry (diodes D.sub.1 & D.sub.2, resistors R.sub.1 & R.sub.2, and capacitors C.sub.2 & C.sub.3) operates as a high voltage doubler and provides rectification and filtering. Electrical isolation and energy limiting are achieved by use of a shunted core transformer. During normal operation, nearly all flux generated by the AC primary winding goes through core leg which couples the high voltage secondary winding, and energy is delivered to the load. Very little flux flows through the core shunt because of the high reluctance of this path in comparison with a secondary leg of the core. However, when arcing occurs, the energy stored in the collecting cell will discharge. The resistance of the secondary leg increases and nearly all the flux is shunted around the secondary leg.
While this circuit provides reasonably effective overload protection, it has certain disadvantages due to its cost, physical size and power consumption. In particular, the shunted transformer is bulky and expensive due to the size of the closed loop core and the number of windings needed to step up the 60 Hz, AC input voltage at the primary winding.
A number of short-circuit protection circuits employing electronic switching devices are known in the prior art. For example, U.S. Pat. No. 3,243,725 to Raposa et al discloses a voltage converter having as oscillator circuit employing a pair of push pull transistors and a pair of tapped transformers including a saturable transformer.
U.S. Pat. Nos. 3,772,853 to Burge et al, U.S. Pat. No. 3,877,896 to Muskovac and U.S. Pat. No. 4,061,961 to Baker relate to prior art devices for supplying power to electronic precipitators. Also of interest are the power supply circuits shown in U.S. Pat. No. 3,928,793 to Waltz and on page 36 of the June 1957 issue of Radio and T.V. News. The latter power supply employs a low voltage miniature audio step up transformer. Of more general interest are U.S. Pat. No. 4,007,413 to Fisher et al directed to a self oscillating converter circuit and U.S. Pat. No. 4,318,164 to Onodera et al directed to a high frequency switching circuit.
It is an object of the present invention to provide a power supply with automatic short circuit protection which is easily and inexpensively fabricated.
It is another object of the present invention to provide a high voltage power supply employing a high frequency step-up transformer with a small, light weight core.
It is another object of the present invention to provide a power supply with automatic short circuit protection employing a high frequency electronic oscillator and a single high voltage, high frequency transformer.
It is another object of the present invention to provide a power supply with automatic arcing control and short circuit protection which is small in size and weight and consumes a minimum of electric power.
These and other objects and features of the invention will become apparent from the claims and from the following description when read in conjunction with the accompanying drawings.